The present invention relates to a tuning fork crystal oscillation plate and a method of manufacturing the same.
A crystal oscillator incorporating a tuning fork crystal oscillation plate as one of electronic components is mounted and used as a reference signal source or clock signal source in an electronic device such as a computer, portable telephone, or a compact information device. Strong demands for downsizing, lower profile, and cost reduction have been conventionally imposed on the crystal oscillator.
The arrangement of a conventional tuning fork crystal oscillation plate will be described with reference to FIGS. 9 and 10.
In a conventional tuning fork crystal oscillation plate 101, various types of electrodes, e.g., oscillation electrodes, frequency adjustment electrodes, and external connection electrodes to electrically connect to a packaging container are formed on the surface of a crystal oscillation piece 101a having an tuning fork-like outer shape. The tuning fork crystal oscillation plate 101 is fabricated by photolithography, chemical etching, or the like.
The tuning fork crystal oscillation piece 101a comprises a base 102, and first and second oscillation arms 103a and 103b extending in the same direction from one side of the base 102.
The first oscillation arm 103a has grooves 104a and 104c in its front and rear main surfaces, respectively. The second oscillation arm 103b has grooves 104b and 104d in its front and rear main surfaces, respectively. The grooves 104a and 104c of the same oscillation arm 103a, or the grooves 104b and 104d of the same oscillation arm 103b are generally formed at opposing positions. The tuning fork crystal oscillation piece 101a has a thickness of 100 μm, and each of the grooves 104a to 104d has a depth of about 30 μm.
The groove 104a formed in the front main surface of the first oscillation arm 103a has a first front main surface electrode 106a on its inner surface, and the groove 104c formed in the rear main surface of the oscillation arm 103a has a first rear main surface electrode 107a on its inner surface. The first oscillation arm 103a has first side surface electrodes 108a and 108b on its two side surfaces.
The groove 104b formed in the front main surface of the second oscillation arm 103b has a second front main surface electrode 106b on its inner surface, and the groove 104d formed in the rear main surface of the oscillation arm 103b has a second rear main surface electrode 107b on its inner surface. The second oscillation arm 103b has second side surface electrodes 109a and 109b on its two side surfaces.
The front main surface electrode 106a and rear main surface electrode 107a of the first oscillation arm 103a, and the side surface electrodes 109a and 109b of the second oscillation arm 103b are electrically connected to each other and lead to an external connection electrode 111a through a connection electrode 112a to provide one terminal.
The front main surface electrode 106b and rear main surface electrode 107b of the second oscillation arm 103b, and the side surface electrodes 108a and 108b of the first oscillation arm 103a are electrically connected to each other through a connection electrode 112b and lead to an external connection electrode 111b to provide the other terminal. Hence, the tuning fork crystal oscillation plate 101 has a two-terminal arrangement.
An alternating voltage is applied between the two terminals. In a momentary state shown in FIG. 10, the side surface electrodes 108a and 108b of the first oscillation arm 103a are set at a + (positive) potential, and the front main surface electrode 106a and rear main surface electrode 107a of the same are set at a − (negative) potential. An electrical field is generated from + to −. These polarities are reversed in the second oscillation arm 103b. These electrical fields generate expansion and contraction in the oscillation arms 103a and 103b made of a crystal material to flex them.
A conventional method of manufacturing a tuning fork crystal oscillation plate will be described with reference to FIGS. 11A to 11F. FIGS. 11A to 11F show a series of steps of forming the conventional tuning fork crystal oscillation plate 101 shown in FIGS. 9 and 10 by means of formation of the outer shape of the tuning fork crystal oscillation piece 101a and the grooves 104a to 104d, and pattern formation of the electrodes 106a, 106b, 107a, 107b, 108a, 108b, 109a, and 109b. 
First, a two-layer film consisting of a Cr film 151 and Au film 152 is formed as a corrosion-resistant film on each of the front and rear main surfaces of a crystal substrate 150 having a thickness of 100 μm. After applying a photosensitive resist film on each corrosion-resistant film, the crystal substrate 150 undergoes exposure and development which are necessary for formation of the outer shapes of a plurality of tuning fork crystal oscillation pieces 101a, to obtain each outer shape pattern 153 formed of the photosensitive resist film (FIG. 11A). As the photosensitive resist film, a positive film is used.
The exposed Au films 152 and Cr films 151 on which the outer shape patterns 153 do not exist are sequentially chemically etched. Hence, the crystal substrate 150 is exposed in the regions at the boundaries of the tuning fork crystal oscillation pieces 101a. 
The photosensitive resist films that form the outer shape patterns 153 are not removed as they are not exposed. Then, the outer shape patterns 153 are exposed and developed with groove-shape patterns to obtain outer shape patterns 153a having openings corresponding to the groove portions. The exposed portions of the Au films 152 are chemically etched to obtain Au films 152a having openings corresponding to the groove portions. This exposes the surfaces of Cr films 151a corresponding to the groove portions (FIG. 11B). The chemical etching takes place in an atmosphere that does not expose the photosensitive resist films.
The crystal substrate 150 is dipped in a liquid mixture of hydrofluoric acid and ammonium fluoride to etch its exposed portions, thus obtaining the outer shapes of the plurality of tuning fork crystal oscillation pieces 101a. With this step, in the crystal substrate 150, an outer frame supports the plurality of tuning fork crystal oscillation pieces 101a with a support beam. In this step, etching is stopped before each tuning fork crystal oscillation piece 101a reaches a desired shape.
After that, the Cr films 151a are etched using the photosensitive resist films 153a and Au films 152a as masks. Hence, Cr films 151b having openings corresponding to the groove portions are obtained, and the surface portions of the crystal substrate 150 corresponding to the openings are exposed (FIG. 11C).
When etching the crystal substrate 150 again by dipping it in the liquid mixture of hydrofluoric acid and ammonium fluoride, grooves 104a to 104d are formed in each tuning fork crystal oscillation piece 101a, and the outer shape of each tuning fork crystal oscillation piece 101a is completely etched into a desired shape. Each of the grooves 104a to 104d has a depth of approximately 30 μm to 40 μm.
The photosensitive resist films 153a, Au films 152a, and Cr films 151b are sequentially removed. Except for the outer frame and support beam, only the tuning fork crystal oscillation pieces 101a remain. The grooves 104a to 104d are formed in two oscillation arms 103a and 103b of each tuning fork crystal oscillation piece 101a (FIG. 11D).
A metal film 154 to be used as electrodes is formed on the entire surface of each tuning fork crystal oscillation piece 101a including the inner portions of the grooves 104a to 104d. As the metal film 154, generally, a two-layer film of Cr and Au, an aluminum film, or the like is used, and formed by sputtering or vacuum deposition. Assume that a two-layer film of Cr and Au is employed in this case. The total thickness of the two layers is about 100 mm.
A photosensitive resist film 155 is formed on the entire surface of the metal film 154 of the tuning fork crystal oscillation piece 101a (FIG. 11E) by coating. An electrodeposition resist is used for three-dimensional coating of the photosensitive resist film 155.
To obtain the shapes of the various types of electrodes shown in FIGS. 9 and 10, exposure and development required for patterning the photosensitive resist film 155 are performed. During the exposure, if the mask of the exposure apparatus comes into contact with the crystal substrate 150, the tuning fork crystal oscillation piece 101a may undesirably drop from the crystal substrate 150 due to the resultant impact. For this reason, a non-contact double-sided projection exposure apparatus is used as the exposure apparatus.
The metal film 154 is etched using the remaining photosensitive resist film as a mask, and thereafter the photosensitive resist film is removed. Thus, various types of electrodes including front main surface electrodes 106a and 106b, rear main surface electrodes 107a and 107b, and side surface electrodes 108a, 108b, 109a, and 109b are obtained, and the outer shape of the tuning fork crystal oscillation plate 101 in which the various types of electrodes are formed on the surface of the tuning fork crystal oscillation piece 101a is completed (FIG. 11F).
After that, metal films 110a and 110b formed at the distal ends of the oscillation arms 103a and 103b are increased or decreased to adjust the oscillation frequency. To form the metal films 110a and 110b, Au or Ag is used. Trimming of the metal films 110a and 110b employs a laser method or ion etching method. Frequency adjustment is performed after packaging the tuning fork crystal oscillation plate 101 in a container, or after packaging the tuning fork crystal oscillation plate 101 in a container and sealing a tuning fork crystal oscillation plate packaging space with a lid.
Reference 1 (U.S. Pat. No. 3,969,641), reference 2 (Japanese Patent Laid-Open No. 52-61985), reference 3 (Japanese Patent Laid-Open No. 56-65517), reference 4 (WO00/44092), reference 5 (Japanese Patent No. 3,729,249), reference 6 (Japanese Patent Laid-Open No. 2004-159072), and the like disclose the conventional tuning fork crystal oscillation plate 101 as described above and its manufacturing method.
The conventional tuning fork crystal oscillation plate 101 and its manufacturing method have the following problems.
(1) In the conventional tuning fork crystal oscillation plate 101, the grooves 104a and 104c are formed in the front and rear surfaces of the oscillation arm 103a to oppose each other, and the grooves 104b and 104d are formed in the front and rear surfaces of the oscillation arm 103b to oppose each other. Thus, with the conventional manufacturing method, variations occur in the outer shapes of the tuning fork crystal oscillation pieces 101a and the shapes of the grooves 104a to 104d, in the forming positions of the grooves 104a to 104d, depending on the alignment accuracy of the front and rear main surfaces of the crystal substrate 150 of the exposure apparatus, and in the depths of the front grooves 104a and 104b and the rear grooves 104c and 104d due to etching from the front and rear sides. These variations appear as variations in frequency among the respective tuning fork crystal oscillation plates 101. More specifically, the frequency may largely differ between, of the plurality of tuning fork crystal oscillation plates 101 formed from one crystal substrate 150, one obtained from the central portion of the crystal substrate 150 and one obtained from near the peripheral portion of the crystal substrate 150. Also, variations in frequency may increase among crystal substrate lots. More specifically, when compared to a tuning fork crystal oscillation plate having no grooves 104a to 104d, the variations in frequency were three times. When the variations in frequency increase among the tuning fork crystal oscillation plates 101 in this manner, the frequency adjustment amount by using the metal films 110a and 110b in the tuning fork crystal oscillation plate 101 undesirably becomes very large.
(2) If an error occurs in the shapes of the grooves 104a to 104d, e.g., if a shift occurs between the forming positions of the front grooves 104a and 104b of the oscillation arm 103a and the rear grooves 104c and 104d of the oscillation arm 103b, an oscillation frequency differs between the two oscillation arms 103a and 103b, leading to an imbalance. In this case, leaking oscillation is conducted to the base 102. Consequently, a crystal impedance (to be referred to as CI hereinafter) value becomes large and unstable.
One of factors that cause these two problems is as follows. In a general double-sided projection exposure apparatus used for the manufacture of the conventional tuning fork crystal oscillation plate 101, the alignment accuracy of the front and rear surfaces is about several μm, and the resolution is on the 5-μm level. With the conventional manufacturing method, the outer shape of the tuning fork crystal oscillation piece 101a and the grooves 104a to 104d must be formed by two separate exposure operations. If the accuracy error of the exposure apparatus for each exposure is worst, it leads to the accuracy variations of several μm×2. These variations appear not only as vertical and horizontal shift but also as shift in the rotational direction. Added with exposure to form electrodes, exposure must be performed three times, leading to further large variations in positional accuracy.
The double-sided projection exposure apparatus is comparatively expensive. With the conventional manufacturing method, the yield is low to increase the manufacturing cost.
With the conventional manufacturing method, exposure must be performed three times, as described above, leading to a large number of steps.
Another problem of the conventional manufacturing method will be described with reference to FIGS. 12A to 12C. In the step described in FIG. 11C of etching the Cr films 151a, the presence of the Au films 152a on the Cr films 151a causes cell corrosion. The Cr films 151 are etched within a shorter period of time (more specifically, within several ten sec) when compared to a case wherein no Au films 152a are present, and the etching amounts of the Cr films 151 are also difficult to control. Therefore, in practice, side etching increases as shown in FIG. 12A, and the Cr film patterns 151c become thinner than desired pattern shapes. This problem leads to large differences among the crystal substrates 150. When etching the crystal substrate 150 using the thin patterns 151c, steps 156 are formed on the two sides of each of the oscillation arms 103a and 103b, as shown in FIG. 12B. As the Cr films 151 are etched a total of two times, i.e., the step of FIG. 11B and the step of FIG. 11C, the shape variations in Cr film patterns 151c and in oscillation arms 103a and 103b become large. The section of a finished product including these influences is as shown in FIG. 12C, and may not be as shown in FIG. 11F. In FIGS. 12B and 12C, reference numerals 101a′, 104a′ and 104c′, 106a′, 107a′, and 108a′ and 108b′ respectively denote a modified tuning fork crystal oscillation piece, modified grooves, a modified front main surface electrode, a modified rear main surface electrode, and modified side surface electrodes, respectively.